576 
PROFESSOR OWEN ON THE MEGATHERIUM. 
between the first alveoli. The extent of the palatal part of the maxillary in advance 
of these alveoli is about 1 inch to the hindmost part of the premaxillary, 22, and 2^ 
inches to the apex of the process (21) articulating with that bone. 
The premaxillaries (Plates XXIV., XXI. fig. 1, and XXII. figs. 1 & 2, 22) have coa- 
lesced along the major part of their extent, leaving only a median fissure on their upper 
surface (Plate XXII. fig. 2, 22), of about 1^ ineh in length, at about the same distance 
from their slightly expanded anterior ends ; at their under surface (Plate XXIV.) 
the same fissure is more advanced, and contracts to a few foramina. They form a 
slender, elongated, subdepressed, four-sided portion of bone, and constitute a singular 
anterior termination of the skull. 
At the base or back part this portion of bone measures 4^ inches across ; the fore- 
end is 2 inches 9 lines across ; the narrowest part, near this end, is 2 inches 4 lines 
across ; the vertical diameter is pretty nearly throughout 1 inch 6 lines, but decreases 
anteriorly. The posterior third of the bone sends upwEird from its median line a 
ridge, which enlarges as it approaches the corresponding ridge from the maxillaries, 
and there presents a smooth and gradually expanding groove at its upper part, for 
the support of the vomer or its cartilaginous septal prolongation (Plate XXIII. fig. 2). 
Anterior to the median ridge begins the groove which sinks into the fissure, and is 
then again continued forward as a groove to within an inch of the fore-end of the 
bone ; this part (Plate XXIV. 22) seems crossed by a rough plate or cap of bone, flat, 
and about an inch in breadth at its upper part, and there terminating behind, as it 
does below, in a free margin. 
The under surface of the premaxillary mass (Plate XXIV. 22) is rather convex antero- 
posteriorly, as also transversely along its middle third : the groove indicating the 
primitive suture runs along the whole of this surface, and sinking into its fore-part, 
opens by two or three foramina into the fissure which is seen on the upper surface. 
The baek part of the under surface of each premaxillary is notched to receive a 
triangular process of the palatine part of the maxillary (21): the more slender median 
parts of the notehes partly divide the prepalatal or incisive fissure (a), which thus 
presents the form of a chevron. 
The malar (Plate XXI. fig. 1,26) is a singularly developed mass of bone, and has 
always attracted attention as one of the most remarkable features of the skull, 
from the period of the earliest notices of the Megatheriuin. Its bulk and complex 
shape appear to relate to the unusual share which a modified and largely developed 
inasseter muscle must have taken in the aet of mastication. 
Firmly articulated by extensive reciprocally indented sutures, at one end with the 
maxillary (31), at the other end with the zygomatic bones (27), and giving an extensive 
surface of attachment, by a peeuliar upward prolongation, to fasciculi of the temporal 
muscle, it afforded the requisite fixity for the origins of the large and complex mas- 
seter. 
The suture with the maxillary is in great part obliterated in the skull under descrip- 
